1. Field of the Invention
The present invention relates to an image forming apparatus of the electrophotographic type and its high voltage power source device.
2. Related Background Art
Hitherto, an image forming apparatus of the electrophotographic type (copying apparatus, printer, or the like) is constructed as shown in, for example, FIG. 5. In the diagram, reference numeral 1 denotes a controller to control an electrophotographic processing sequence and has a CPU; 2 indicates a high voltage power source device; 3 indicates a drum as a photosensitive member; 4 indicates a roller as a charging member; 5 indicates a scanner to scan using a laser beam; 6 indicates a reflecting mirror of the laser beam; 7 indicates a developing unit having a toner carrier; 8 indicates a transfer charging unit; and 9 indicates a cleaner.
In the charging of such an image forming apparatus, in the case where the charging member 4 is directly come into contact with the photosensitive member 3 and the photosensitive member 3 is charged as shown in the diagram, an output which is obtained by directly multiplexing an AC bias to a DC bias is used as a high voltage output that is applied to the charging member 4. One of the above charging means has already been proposed by the same applicant as that of the present invention as shown in U.S. Pat. No. 4,851,960. In this instance, since a potential of the DC bias is set to the surface potential of the photosensitive member 3, the DC bias is subjected to a constant voltage control. The voltage at this time is equal to a voltage according to a concentration within a range of about -750 V to -600 V. The AC bias is used to efficiently charge the drum. Although a proper AC voltage is needed for charging, in the case where the AC voltage is too high, there is a drawback in that an electric breakdown of the photosensitive member 3 may occur. It is, accordingly, necessary to control the voltage to a proper voltage value. Although the optimum condition of the AC voltage which is applied to the charging member 4 varies depending on the environment, a voltage within a range of about 1600 V.sub.pp (peak to peak) to 2000 V.sub.pp is optimum. A frequency of the AC bias is set to a frequency within a range of about 100 Hz to a few kHz. Such a frequency is substantially determined by a processing speed of the electrophotographic apparatus. However, since a sound of a frequency that is twice as high as the AC frequency is generated by the charging member 4 and the frequency of such a generated sound lies within an audible range (that is, it is accompanied by noise), it is therefore, necessary to set the frequency as low as possible. On the other hand, even when the frequency is too high, a good charging state cannot be obtained. To suppress the noises as much as possible, an ordinary sine wave is used for the purpose of reduction in harmonics. An output which is obtained by multiplexing the AC bias to the DC bias is used as a high voltage output which is applied to the developing unit 7 for obtaining a visible image from an electrostatic latent image on the photosensitive member 3 by using a developing agent as shown in a developing method disclosed in U.S. Pat. No. 4,292,387. In this instance, an output voltage of the AC bias within a range of about 1200 V.sub.pp to 1700 V.sub.pp is used. A frequency is set to about a few kHz.
In the above conventional example, however, there is a drawback in that an AC voltage which is used for charging interferes with an AC voltage which is used for development and an interference fringe corresponding to waviness of both of the frequencies is formed in the image, so that such an interference fringe typically appears on the image formed depending on the set state of the frequency. On the other hand, the surface potential which is charged onto the photosensitive member is influenced by the AC bias and even when the DC bias generates a predetermined voltage, a slight potential difference is caused on the surface potential. Therefore, a fringe of the potential difference which is influenced by the frequency of the AC bias is produced on the photosensitive material. In a printer of the electrophotographic type, since the photosensitive member is exposed by dots of a light source, waves between the dots which are influenced by a print density of the light source for exposure are produced on the photosensitive member due to a predetermined frequency. In this instance, in the case where a frequency of the waves of the image which are produced on the photosensitive member by a combination of the print density of the light source and the image which is produced is close to the frequency of the high voltage AC bias for charging, a phenomenon such as a moire occurs on the image produced. FIG. 6 shows the above relation. FIG. 6 shows an AC frequency for charging at which a moire occurs to the print density. To avoid such a moire, the AC bias must be driven at a frequency according to the print density. In the case of efficient charging, it is necessary to set the frequency of the AC bias to a frequency within a range of about 100 Hz to 1 kHz, and it is also necessary to use a sine wave in order to reduce the harmonics at the audible frequencies because of the relation of the noise frequencies. A frequency at which the moire is prevented differs in dependence on each print density and the charging operation must be efficiently executed. It is, therefore, difficult to switch the print density in the same apparatus.
One means for solving the above problems has already been proposed by the same applicant as that of the present invention by Japanese Patent Application Laid-Open No. 4-66973. In this case, however, the frequency is set to a value which is out of integer-time relation in order to prevent an interference. It is, therefore, necessary to adjust the frequency so as not to set the frequency to a value that is an integer times as high as another frequency for each apparatus. Further, it is necessary to set the frequency so as to avoid such an integer-time relation. There is a case where the optimum combination cannot be obtained due to the frequency relation between two high voltages. Particularly, in the case where it is necessary to have a frequency dividing ratio of an odd-number of times, a duty of the output signal is not equal to 50%. Such a situation becomes a factor to generate an unnecessary DC component in the output voltage and there is a case where the picture quality is unexpectedly deteriorated.